Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear Power Engineering", McGraw-Hill Book Company, Inc., 1962.
In a known type of nuclear power reactor, for example, as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core, housed in a pressure vessel, is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle as shown, for example, in U.S. Pat. No. 3,431,170. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
If the reactor core is operated at a high power density, forced recirculation of the coolant through the core is necessary for adequate heat removal.
A number of coolant recirculation arrangements are known. As shown by J. M. Roberts in U.S. Pat. No. 3,378,456 and by D. E. Hughes in U.S. Pat. No. 3,389,055, it is known to recirculate coolant through the reactor core by the use of jet pumps mounted in an annular space between a shroud surrounding the nuclear core and the inside of the pressure vessel. The jet pumps take coolant from the region above the core and pressurize the coolant in a plenum in the lower portion of the pressure vessel beneath the fuel core from whence it flows upward through the fuel assemblies of the core.
R. O'Neil in U.S. Pat. No. 3,366,548 shows a similar arrangement of jet pumps which are driven by the incoming feedwater. While this purports to eliminate external recirculation piping loops, it creates an interdependence between feedwater and recirculation flow which is impractical where variable recirculation flow rate is used to control reactor power level.
In another known coolant recirculation arrangement, motor driven rotary impellers, mounted within the pressure vessel in alignment with the annular space surrounding the nuclear core provide the forced recirculation of the coolant.
Such arrangements are shown for example by Kornbickler et al in U.S. Pat. No. 3,467,578; by Leime et al in U.S. Pat. No. 3,723,247; and by Yoshimoto et al in U.S. Pat. No. 4,315,800 (the foregoing being incorporated herein by reference). The impeller drive motor may be mounted outside the pressure vessel (as shown in Kornbickler et al FIG. 1a) or the drive motor may be located inside the pressure vessel (as shown in Kornbickler et al FIG. 1b).
Such rotary pump arrangements eliminate external recirculation flow loops while retaining (by drive motor speed control) the capability of variable recirculation flow rate.
In the above-mentioned patents, the nuclear core is surrounded by a shroud which with the inner wall of the pressure vessel forms the downcomer annular space for coolant recirculation flow. In the vessel construction, the shroud supports the lateral core support structures and an array of steam separators above the core (as shown, for example, by the previously mentioned U.S. Pat. No. 3,378,456). Therefore, the shroud is subjected tc relatively high loads, particularly dynamic loads such as seismic loads.
It is an object of the invention to eliminate the core shroud structure in a nuclear reactor.
It is another object to reduce the dynamic loads carried by the nuclear core support structure.
It is another object to reduce the diameter of the pressure vessel for a given size nuclear core.
It is a further object to replace the function of the core shroud as a downcomer for coolant recirculation flow with tubular coolant conducting conduits.